The device of the present invention relates to detector dewar assemblies used in thermal imaging systems.
In most thermal imaging systems using semiconductor detection devices which must be cryogenically cooled, it is necessary to provide a housing (or "dewar") for the detector device. The dewar must thermally isolate yet provide electrical connections between the detector array and the housing exterior so that the array may be electrically interfaced to other signal processing subassemblies.
Typically, these dewars include several concentric cylindrical tubes or housings. The inner tube is generally comprised of a long cylindrical glass bore which opens to the base of the dewar. This bore, often referred to as the "coldwell", is multifunctional. First, the detector device or array is mounted at the top of the coldwell (the "endwell"). A "cold finger" or cryogenic cooling device is generally inserted into the bore of the coldwell to effect cooling of the detector array. Often the surfaces of the coldwell within the vacuum space are metalized and then etched or delineated in order to define conductive leads which run the length of the bore.
Past dewars have used a gold plated glass bore to provide a thermally reflective shield which reduces radiation heat load. In addition, glass has been used because it has the advantage that it is both a good electrical insulator and a poor thermal conductor. Moreover, glass is not ductile and once fixed in the system, alignment will remain true unless the bore breaks. However, the glass coldwell has the disadvantage that it is extremely fragile and requires complex and expensive assembly techniques.
In some prior art dewars, the base portion of the coldwell supports a flange to which a cap is attached. This cap is typically made of some metal which has appropriate structural and thermal properties and can maintain a hard vacuum. The top part of the cap directly above the detector array holds a transmissive window.
One type of dewar is shown in the article entitled "Automatic Testing Of Infrared Detector Arrays", D. A. Jones, SPIE, Vol. 344, Infrared Sensor Technology (1982). In such a dewar, the flange comprises a monolithic ceramic disc with a center through hole. The upper surface of such a ceramic disc includes a radial pattern of electrically conductive paths from the inside diameter to the outside diameter. The delineated conductors from the detector device that are on the coldwell are connected via jumper wires to the inside diameter portion of the conductor paths on the ceramic disc. The opposite end of the conductive paths on the outside diameter portion of the ceramic disc connect to a pin assembly covering substantially all of the disc outside the vacuum chamber. A tape cable connects these pins to a bias resistor board or pack and/or an external connector located on the protective dewar housing. A fused dielectric ring covers the conductive paths on the inner portion of the ceramic disc providing an electrically insulated, vacuum tight surface. A metal ring is fused onto the dielectric ring for soldering purposes. Similarly, a metalization is deposited on the other side of the ceramic disc. The upper vacuum cap is attached to the ceramic by soldering and the lower base is attached to the ceramic by brazing to the metalized ring.
The above described design has some disadvantages. The attachment of the upper cap to the dielectric surface of the monolithic ceramic forms a relatively weak joint which is easily damaged during handling. Also, the use of conductive paths on the surface of the ceramic disc tends to create, in combination with the dielectric fusing process, a joint which may develop vacuum leaks. Further, the dispersed pin configuration makes in-process electrical testing difficult because the inner-most pins are not readily accessible.
In a second dewar design, a metal flange, bonded appropriately to the glass coldwell, serves primarily as a supporting member for the vacuum chamber cap and window. The base portion of the glass coldwell is thickened so that it may include buried conductive wires which surface above and below the flange providing an electrical exit from the vacuum chamber. Gold wires are used to connect the axial leads on the coldwell to both the detector array and to the surfaced conductors on the coldwell above the flange. The conductors surfacing below the flange (outside the vacuum) may be connected to a conventional tape cable or other wiring device.
In either of these designs, a second larger flange may be connected to the coldwell at its base. This flange supports an outer protective housing which encloses the vacuum chamber, as well as supporting the mounting flange for the dewar. The housing has openings for the end of the vacuum cap and the electrical connectors. In either design, a tape cable or other wiring device connects from the vacuum exit conductors to standard electrical interconnects. This tape cable generally takes the form of a copper conductor pattern electrodeposited on a Polyimide Kapton sheet. The tape cable is usually multilayered.
These prior designs have several disadvantages. First, to repair the detector array or other connections within the vacuum chamber, the entire device must be disassembled; the outer housing and the vacuum cap removed. Second, since the coldwell is made of glass, the device is particularly susceptible to breakage during assembly, disassembly, and normal in-house and end item use. Third, the gold jumper wires used at the array, flange and at the buried lead connections are susceptible to breakage. Fourth, the volume of the vacuum chamber is usually a small percentage of that of the finished assembly, such that when materials outgas into he vacuum chamber, the vacuum degrades at a high rate. Finally, because the design uses many parts and requires use of complex assembly techniques, the cost per unit is usually high.
Accordingly, it is a primary object of the present invention to provide an improved detector dewar assembly.